This is a study of the way in which normal human subjects control posture and balance while upon a platform and performing simple stance and locomotion tasks. The platform is movable and can introduce, unexpectedly to the subject, various kinds of movement perturbations. During the postural adjustments which result from the perturbations, sensory feedback information from the ankle joints (proprioceptive and cutaneous inputs) and from visual inputs can be unexpectedly modified, thereby forcing the subject to adapt his postural control strategy to these conditions. These methods are used to quantify the independent roles of proprioceptive, vestibular, and visual inputs to postural and balance controls. The research is designed to describe the organization of EMG activities among the muscles of the legs and to define the functional significance of stereotyped response patterns in the task of posture control. Patients with clinically identified neurological deficits are also studied and the results compared to normal subjects. These data are used to develop more quantitative descriptions of spacticity, ataxia, and vestibular imbalance in terms of the organizational characteristics of postural-balance controls.